The present disclosure generally relates to frequency control devices, and more particularly to spur reduction in a Phase-Locked Loop (PLL).
A phase-locked loop (PLL) circuit, as is generally known in the art, finds application where circuit synchronization is needed. In modern digital receivers, the phase-locked loop circuit may not be a hardwired implementation, but may rather be software, or microprocessor based implementation. The phase-locked loop will generally include three basic components. A phase detector, a loop filter and a voltage-controlled oscillator. As the incoming signal and the generated signal change with respect to each other, the phase difference or error is used as a time-varying signal into the loop filter. The loop filter generally governs the response of the phase-locked loop to the variations in the error signal. A well designed phase-locked loop will be able to track changes in the phase of the incoming signal, but will not be overly responsive to noise in the receiver.
The purity of the frequency output of an integrated phase-locked loop is an important factor in many applications. The clock used in the digital logic can couple through parasitics and surges to other parts of the phase-locked loop. These disturbances can result in spurious tones at the output of the phase-locked loop, also referred to herein as RF “spurs.”
Spurs are generally reduced through the careful selection of phase-locked loop parameters, such as loop bandwidth, layout and signal routing. Differential logic can also be employed to reduce the ripple on the power or voltage supply lines. Spur reduction can also be achieved by shielding. However, the amount or spur reduction typically resulting from shielding is minimal. Reductions in the loop bandwidth are not preferable, as reducing the bandwidth can increase the lock time of the phase-locked loop and make it harder to integrate the loop filter. The contribution of noise from the voltage controlled oscillator can also increase.
Accordingly, it would be desirable to provide a system that reduces coupling induced spurs and addresses at least some of the problems identified.